This invention relates to a process for making expandable styrene polymer particles non-lumping on pre-expansion.
The making of low density, cellular, shaped, plastic articles from expandable particles of styrene polymers is well-known. Such particles generally contain a blowing agent which boils below the softening point of the polymer and which will cause the particles to expand when they are heated.
The particles are generally pre-expanded before introduction into the mold to provide better fusion and less density variation in the molded article. Such a pre-expansion is described in U.S. Pat. No. 3,023,175 and U.S. Pat. No. 3,577,360.
These pre-expanded particles are placed into a mold cavity which defines the shape of the desired finished articles. The particles are heated above their softening point, whereupon the particles expand to fill the mold cavity and fuse together.
An undesirable result of the pre-expansion before introduction into the mold is the tendency of the pre-expanded particles to clump together and form lumps which render the particles unsuitable for molding. These lumps cannot be properly conveyed in processing plants and are unsuitable for charging into molds of intricate contour due to improper fill-out of the molded articles. Even if reasonable fill-out is achieved, variations in density within the molded article can result and voids also occur.
Various methods have been proposed for the prevention of lumping during pre-expansion, but these have all suffered from some objectionable feature such as rapid loss of blowing agent, poor flow of the treated beads, dust problems which cause plugging of the steam ports, and often serious reduction in fusion of the particles on molding. U.S. Pat. No. 3,520,833 teaches the addition of lecithin during the impregnation of the particles with the blowing agent. Unfortunately, the lecithin imparts an undesirable odor to the molded articles. U.S. Pat. No. 3,462,293 teaches to coat the particles with polymeric materials by a fluid bed process. This process involves an additional expense of fluidizing the particles and coating with the polymer latexes. U.S. Pat. No. 3,444,104 teaches the addition of calcium silico aluminate. This additive tends to plug the equipment.